1. Field of the Invention
This invention relates to a system to prevent a storage battery intended to provide starting power for an engine from being excessively discharged by load circuits that remain connected across the battery when the engine is not running. In particular, it relates to a protection system to disconnect load circuits from the storage battery of an engine-driven vehicle when the engine is not running and when current through the load circuits causes the battery to become so discharged as to lower its voltage below the value necessary to restart the engine.
2. The Prior Art
Various systems have been used to prevent a storage battery that provides starting current for a vehicle and load current for lights, horns, radios, and other load circuits from being discharged by the load current to a level below that necessary for the battery to restart the vehicle. Such protection systems have typically operated by disconnecting the load circuits from the battery when the charge in the battery drops below a predetermined value. In some systems, the value of the retained charge is considered to be indicated by the terminal voltage of the battery. However, when the engine is running, it is important that the protection system not interfere with normal operation of the vehicle, such as by disconnecting its lights, even if the retained charge drops below the restart level.
Systems discussed by Rickey in U.S. Pat. No. 3,474,296; Frezzolini et al. in U.S. Pat. No. 3,656,045; Ciarnello et al. in U.S. Pat. Nos. 4,088,940 and 4,137,557; and Sheldrake in U.S. Pat. No. 4,493,001 fail to provide a control switch connected to a protection circuit to keep the protection circuit out of operation when the control switch is closed (corresponding to having the engine running), thus allowing current to reach load circuits as long as the switch is closed, whether or not the battery voltage is high enough to restart the engine.